Negative Magnetization and Exchange Bias Behavior in Nanocrystalline Ho2CoMn1−xNixO6 (x=0‐0.4) Double Perovskite

Abstract

A gel combustion method is opted to prepare nanocrystalline samples of Ho2CoMn1−xNixO6 (x = 0–0.4) (HCMNO) double perovskite. Structural studies reveal a formation of nanocrystalline samples, which crystallize into monoclinic structure with space group P21/n. A drop in the lattice parameters including unit cell volume are noticed with Ni substitution. Magnetization versus temperature (M-T) analysis shows a single FM transition in the parent compound at 83 K (TC1) and it is ascribed to the super exchange interaction in Co2+ - O2- - Mn4+ networks. However, it is turned into two FM transitions (TC1, TC2) after Ni substitution, where TC2 arises due to possible super exchange interaction in Co2+ - O2- - Ni4+ networks. The transition temperature TC1, is decreasing from 83 to 76 K for x = 0.0–0.4 sample, whereas TC2 increases up to 88 K for x = 0.4 sample. A magnetization reversal (MR) has been observed in higher Ni concentration (x = 0.3 and 0.4) samples in the field cooled state with a compensation temperature (Tcomp) of 4.4 K and 10 K respectively. This MR is attributed to the competition between the ferromagnetic (FM) moment of the transition metal (TM) ions and the polarization of magnetic moments of rare-earth (RE) ions at low temperatures. A typical bipolar switching of magnetization has been demonstrated for x = 0.3 and 0.4 sample. At 5 K, the hysteresis (M-H) loops demonstrate a decrease in saturation magnetization (MS) and coercivity (Hc) value after Ni substitution. However, the observed MS of the order of 11.60–10.27 µB/f.u. (for x = 0.0–0.4) at 5 K is attributed to some visible contribution related to the polarization/ordering of Ho moment with the net magnetic moment of TM ions. A maximum exchange bias field value of 230 Oe has been observed at 5 K for x = 0.4 sample. It is due to the competition between the spin canted magnetic moments of RE ions and the FM moments of TM ions.